1. Field of the Invention
The present invention relates to a transfer device for transferring plate-shaped objects.
2. Description of the Related Art
The recent trend is that more and more vertical heating furnaces are used, instead of the conventional horizontal heating furnaces, as apparatus for heat-processing semiconductor wafers (hereinafter called "wafers") in the course of manufacturing semiconductor devices. In a vertical heating furnace, a plurality of wafers are heat-processed while being held in a wafer boat, positioned one above another and parallel to one another. The wafer boat is moved into the furnace, thereby loading the wafers thereinto, and moved out of the furnace, thus unloading the wafers therefrom.
The wafer boat is a holding device shaped like a hollow cylinder and is made of, for example, quartz which has high heat resistance and high chemical stability. It comprises, for example, four vertical support rods, each having grooves. The wafer boat holds wafers, each set at its circumferential portion in the four grooves made in the four support rods. Hence, the wafers are held parallel to one another and one above another.
Each wafer undergoes various processes in various apparatuses incorporated in a system for manufacturing semiconductor devices. The wafer is transferred among these apparatuses, loaded into one apparatus and unloaded therefrom. A wafer carrier, a container made of, for example, resin, is used to contain a plurality of wafers and transfer them from one apparatus to another.
Attempts have been made to utilize more rapidly decreasing temperatures in heat treating wafers in order to enhance throughput. One of them is to mount wafers on ring-shaped support plates which are attached to a wafer boat, respectively. More specifically, as shown in FIG. 1, a number of ring-shaped support plates 11 are attached to the support rods 12 of a wafer boat, each having a recessed surface 11a and defining a wafer-mounting level. A wafer W is mounted on the recessed surface 11a of each ring-shaped support plate 11, contacting the plate 11 at its circumferential portion. Heat of the circumferential portion of wafers W can therefore be transmitted outside from the wafer W through the plate 11, and the temperature difference between the circumferential and center portions of the wafer W can be reduced. As a result, the temperature of the wafer W can be uniformly lowered and raised even if the rate at which the wafer W is heated is raised.
Wafers are placed within a wafer carrier and are moved among the apparatus incorporated a system for manufacturing semiconductor devices. After the wafer carrier reaches an apparatus having a vertical heating furnace of the type described above, the wafers are transferred onto a wafer boat. The wafer boat is moved into the vertical heating furnace, loading the wafers into the furnace. The wafers W are heat-processed in the furnace. The wafer boat holding the heat-processed wafers W is removed from the furnace, thus unloading the wafers therefrom. Then, the wafers are transferred from the wafer boat back into the wafer carrier. Thus, the wafers must be transferred between the carrier and the boat, so as to be loaded into and unloaded from each apparatus of the system for manufacturing semiconductor devices.
Hitherto, wafers have been transferred between a carrier and a boat, one by one, by means of a transfer device. The transfer device comprises a plate-shaped fork and a drive mechanism for driving the fork. Driven by the mechanism, the fork is inserted into the wafer carrier or the wafer boat, transferring a wafer between the carrier and the boat. A new type of a transfer device has been developed, which comprises a plurality of forks and a drive mechanism and which can transfer a plurality of wafers at a time.
Either type of a conventional transfer device is disadvantageous. To hold a wafer W, the fork is forwarded into a space below the wafer and then lifted, and moved backward. When the conventional transfer device is used to transfer wafers W onto and from the wafer boat of the type shown in FIG. 1, the fork abuts the ring-shaped support plate 11 as it is moved upwards or downwards.
It is therefore necessary to use, besides the fork, a push-up mechanism for moving a wafer W upward or the ring-shaped support plate 11 so as to position the wafer W at the predetermined level. The fork can move at the level without abutting the ring. A transfer device having such a push-up mechanism is disclosed in, for example, U.S. Pat. No. 5,275,521 (U.S. patent application Ser. No. 07/907,545 filed on Jul. 2, 1992). In this transfer device, a push-up disk, which can be moved up and down, is lifted from under a ring-shaped support plate of a wafer boat, lifting a wafer upward from the fork. Then, the fork is moved away from the wafer, thereby placing the wafer onto the push-up disk. Finally, the push-up disk is lowered, mounting the wafer onto the ring-shaped support plate.
This transfer device, however, transfers wafers between a wafer carrier and a wafer boat, only one at a time. The device inevitably requires a long time to transfer all wafers from the carrier to the boat, or vice versa. Consequently, the throughput is low. In the wafer transfer device which can remove a plurality of wafers from a carrier at a time, a plurality of forks are moved simultaneously. If the aforementioned push-up mechanism is incorporated in this wafer transfer device, it can lift but only one wafer at a time. In other words, the push-up mechanism cannot lift a plurality of wafers held on the forks of the wafer transfer device.